Disaster recovery systems typically address two types of failures: a sudden catastrophic failure at a single point in time, or data loss over a period of time. In both types of failure scenario, updates to volumes may be lost. To assist in recovery of data updates, a copy of data may be provided at a remote location. Such dual or shadow copies are typically made as the application system is writing new data to a primary storage device. International Business Machines Corporation (IBM®), the assignee of the subject patent application, provides the following systems for maintaining remote copies of data at a secondary site, Extended Remote Copy (XRC) and Peer-to-Peer Remote Copy (PPRC). These systems provide a method for the continuous mirroring of data to a remote site to failover to during a failure at the primary site from which the data is being continuously mirrored. Such data mirroring systems can also provide an additional remote copy for non-recovery purposes such as local access at a remote site. These IBM XRC and PPRC systems are described in the IBM publication “Remote Copy: Administrator's Guide and Reference,” IBM document number SC35-0169-02 (IBM Copyright 1994, 1996), which publication is incorporated herein by reference in its entirety.
In such backup systems, data is maintained in volume pairs. A volume pair is comprised of a volume in a primary storage device and a corresponding volume in a secondary storage device that includes an identical copy of the data maintained in the primary volume. Typically, the primary volume of the pair will be maintained in a primary direct access storage device (DASD) and the secondary volume of the pair is maintained in a secondary DASD shadowing data from the primary DASD. A primary storage controller may be provided to control access to the primary DASD and a secondary storage controller may be provided to control access to the secondary DASD. In the IBM XRC environment, the application system writing data to the primary volumes includes a sysplex timer which provides a time-of-day (TOD) value as a time stamp to data writes. The host system time stamps data sets when writing such data sets to volumes in the primary DASD. The integrity of data updates is related to insuring that updates are done at the secondary volumes in the volume pair in the same order as they were done on the primary volume. In XRC and other prior art systems, the cross systems common time stamp provided by the system on behalf of the application program determines and maintains the logical sequence of data updates across any number of data volumes on any number of storage systems. In many application programs, such as database systems, certain writes cannot occur unless a previous write occurred; otherwise the data integrity would be jeopardized. Such a data write whose integrity is dependent on the occurrence of a previous data write is known as a dependent write. For instance, if a customer opens an account, deposits $400.00, and then withdraws $300.00, the withdrawal update to the system is dependent on the occurrence of the other writes, i.e., the opening of the account and the deposit. When such dependent transactions are copied from the primary volumes to the secondary volumes, the transaction order must be maintained to maintain the integrity of the dependent write operation.
Volumes in the primary and secondary DASDs are consistent when all writes have been transferred in their logical order, i.e., all writes have been transferred before the writes which are dependent upon them. In the banking example, this means that the deposit is written to the secondary volume before the withdrawal. A consistency group is a collection of updates to the primary volumes such that dependent writes are secured in a consistent manner. For instance, in the banking example, this means that the withdrawal transaction is in the same or a later consistency group as the deposit; the withdrawal cannot be in an earlier consistency group. Consistency groups maintain data consistency across volumes, storage devices and storage systems. For instance, if a failure occurs, the deposit will be written to the secondary volume before the withdrawal. Thus, when data is recovered from the secondary volumes, the recovered data will be consistent.
A consistency time is a time the system derives from the application system's time stamp to the data set. A consistency group has a consistency time for all data writes in a consistency group having a time stamp equal to or earlier than the consistency time stamp. In the IBM XRC environment, the consistency time is the latest time to which the system guarantees that updates to the second volume are consistent. If all the records in the consistency group are written to secondary volumes during a period of time referred to as a “color period”, then the reported consistency time reflects the latest time stamp of all records in the consistency group. Methods for maintaining the sequential consistency of data writes and forming consistency groups to maintain sequential consistency in the transfer of data between a primary DASD and secondary DASD are described in commonly assigned U.S. Pat. Nos. 5,615,329 and 5,504,861 which are incorporated herein by reference in their entirety.
Within a consistency group, redundant data may be eliminated because only the latest version of a block need be retained. However, when data is spread across multiple components, it may be difficult to coordinate the elimination of redundant data unless the final compilation for each volume is handled by a single backup component. Commonly assigned and co-pending U.S. patent application Ser. No. 10/618,242, entitled “AUTONOMIC LINK OPTIMIZATION THROUGH ELIMINATION OF UNNECESSARY TRANSFERS” and filed on Jul. 11, 2003, is incorporated herein by reference and describes the process of ensuring that a single backup component receives all of the data for a particular volume during a particular color. Additionally, the amount of data received by a backup component may be disproportionately large relative to the amount of data received by other backup components, thereby increasing the time to transfer all of the volumes to all of the backup components.
Thus, a need exists to improve the efficiency of the transmission of consistency groups across multiple storage components.